How Much Paint Do I Need? A Simple Calculator Guide (2026)

By Saad · Published 3 November 2026 · Updated 6 November 2026 · 10 min read · Curated Home Stories

Quick answer: Measure the wall area, subtract doors and windows, and divide by the coverage on the tin, usually about 10 square metres per litre (350 square feet per gallon). Most rooms need two coats, so double the result and round up to the next tin size.

Buying paint is a guessing game for most people, and the guess is usually wrong. Buy too little and you run out halfway through a wall. Buy too much and you waste money on paint that sits in the garage. The calculation is simple, and it takes five minutes. This guide shows how to measure a room, work out the paint you need, and buy the right amount.

The paint formula, explained

The formula for paint is simple. First, work out the total wall area: multiply the length of each wall by the height, and add the walls together. Second, subtract the area of doors and windows: a standard door is about 2 square metres (21 square feet), and a standard window is about 1.5 square metres (16 square feet). Third, divide the remaining area by the coverage on the tin, usually about 10 square metres per litre (350 square feet per gallon). Fourth, multiply by the number of coats, usually two. Finally, round up to the next tin size, because it is better to have a little extra than to run out. Our paint calculator does this calculation for you, including doors, windows and coats.

Try the paint calculator.

How to measure a room

Measure the length of each wall with a tape measure, and measure the height from the floor to the ceiling. Write down the measurements, and multiply the length by the height for each wall. Add the walls together to get the total wall area. Measure the doors and windows, and subtract their area. A standard door is about 0.9 by 2 metres, and a standard window is about 1.2 by 1.2 metres, but measure your own doors and windows, because they vary. If the room has a sloped ceiling, measure the average height. If the room has a fireplace or a built-in wardrobe, subtract the area it covers, or measure around it.

Coverage: what the tin says

Paint coverage is printed on the tin, and it is usually about 10 square metres per litre (350 square feet per gallon) for wall paint, and about 12 square metres per litre for ceiling paint. Coverage varies by paint type, colour and surface: a dark colour over a light wall needs more paint, a rough surface needs more paint, and a first coat on new plaster needs more paint. Check the coverage on the tin you are buying, because it varies by brand. Use the tin's coverage, not a generic figure, for the most accurate result. If you are painting over a dark colour with a light colour, allow an extra coat, because the dark colour will show through.

How many coats do you need?

Most rooms need two coats of paint for an even, durable finish. One coat is rarely enough, because it leaves patchy colour and poor coverage. A first coat on new plaster needs a primer or a mist coat, which is paint diluted with water, before the top coats. A dark wall painted a light colour needs a primer and two coats, or three coats without a primer. A light wall painted a similar colour may need only one coat, but two coats will last longer and look better. Budget for two coats, and buy enough paint for two coats, because running out halfway through the second coat is the most common paint mistake.

Typical paint amounts by room

Use this table as a starting point, then measure your own room, because every room is different.

These figures assume standard ceiling height of 2.4 metres, two coats, and coverage of about 10 square metres per litre. Subtract doors and windows from the wall area before calculating.

Primer, ceilings and woodwork

Primer is a paint that prepares the surface, and it is essential on new plaster, on a dark colour, and on a repaired wall. Use a primer before the top coats, and allow it to dry fully. Ceilings need a separate paint, usually a matt white, and about the same amount as the walls for one room. Woodwork, such as skirting and doors, needs a satin or gloss paint, and a small tin is usually enough for one room. Do not use wall paint on woodwork, because it will chip and peel. Buy the primer, the ceiling paint and the woodwork paint separately, and calculate each one.

Buying paint without waste

Buy paint in the largest tin that you need, because it is cheaper per litre than small tins. Round up to the next tin size, because it is better to have a little extra for touch-ups than to run out. Keep the leftover paint for touch-ups, labelled with the room, the colour and the date. Store paint in a cool, dry place, with the lid on tightly, and use it within a year. If you have a lot of leftover paint, use it for a small project, such as painting a piece of furniture, rather than throwing it away. Paint is expensive, so buy the right amount, not too much and not too little.

Paint mistakes to avoid

The most common mistakes are not measuring the room, not subtracting doors and windows, buying only enough for one coat, and not using primer on new plaster or a dark wall. Other mistakes are painting over a dirty or greasy wall, painting in the wrong light, and using the wrong finish for the room. Measure the room, use the calculator, buy enough for two coats, and prepare the surface properly. Two thin coats always look better than one thick coat, and a room that is measured and prepared properly will use less paint and look better than a room that is guessed.

Key measurements at a glance

These are the standard measurements used by designers for a room like this. They are the same figures used throughout this guide, collected here so you can check them at a glance while you plan.

5 mistakes to avoid

1. Choosing paint from a tiny swatch instead of a large sample.
2. Skipping primer on a dark wall, which wastes paint and leaves patchy colour.
3. One thick coat of paint instead of two thin coats.
4. Hanging a gallery wall without planning the layout on the floor first.
5. Starting electrical, gas, plumbing or structural work without a professional.

Note: measurements and spacing guidance in this article are general design guidance for typical homes, in imperial and metric. Check manufacturer instructions and local building regulations for your specific project, especially for electrical, plumbing and structural work.

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